Using GIS to assess the relationship between land use and water quality at a watershed level

Abstract

Daily water quality data from six U.S. Geological Survey (USGS) monitoring stations along the Great Miami River and digital land use/land cover and elevation data for the same watershed were analyzed. The validity of using electrolytic conductivity as a water quality indicator was examined by correlating conductivity with 199 other water quality variables, 60 of which were significantly correlated with conductivity. Positive correlations existed between conductivity and dissolved materials. Negative correlations existed between conductivity and insoluble or suspended materials. Conductivity was also found negatively correlated with stream discharge, and so were most water quality variables that were positively correlated with conductivity. Geographic Information Systems (GIS) were used for analyzing spatial variations of conductivity and land use make-ups in catchment for monitoring stations. It appears that conductivity is related to not only the percentage of urban land use in the catchment but also the cumulative effect of urban land use from upstream. The evidences imply that conductivity may not be a sensitive water quality indicator of nonpoint pollution sources. Further research is needed to identify if the close relation between conductivity and the urban land use may be explained by a high density of point pollution sources in urban areas.